Since the radar level gauging was developed as a commercial product in the 1970's and 1980's, frequency modulated continuous wave (FMCW) has been the dominating measuring principle for high accuracy applications. An FMCW measurement comprises transmitting into the tank a signal which is swept over a frequency range in the order of a few GHz. For example, the signal can be in the range 24-27 GHz, or 9-11 GHz. The transmitted signal is reflected by the surface of the contents in the tank (or by any other impedance transition) and an echo signal, which has been delayed a certain time, is returned to the gauge. The echo signal is mixed with the transmitted signal to generate a mixer signal, having a frequency equal to the frequency change of the transmitted signal that has taken place during the time delay. Due to the linear sweep, this difference frequency, also referred to as an intermediate frequency (IF), is proportional to the distance to the reflecting surface. The mixer signal is often referred to as an IF signal.
Since the development of FMCW, other types of level gauges have been developed, where also the frequency of a transmit signal is varied within a certain range. Although highly accurate, these FMCW-type radar level gauges may be affected by narrow-band interferences from a number of sources e.g., microwave-resonances in the antenna, microwave-resonance in other parts of the waveguide, low signal amplitude due to turbulent surfaces of the content to be measured, bad assembly of the parts of the radar level gauge or contamination of the waveguides by the contents of the tank.
Consequently there is a need to provide a radar level gauge which is able to adapt to narrow-band interferences from at least the sources mentioned previously.
General Disclosure of the Invention
With regards to the above-mentioned desired properties of a radar level gauge, it is a general object of the present invention to enable improved performance of a radar level gauge by limiting or eliminating disturbance from narrow-band interferences.
The present invention is based upon the realization that by dividing a signal comprising information of the distance to a surface in a tank into a plurality of parts, more conclusive information may be produced instead of lost.
According to a first aspect of the present invention, these and other objects are achieved through a method for providing narrow-band interference unaffected measurement of a distance to a surface of a product kept in a tank, which method comprises transmitting electromagnetic transmit signal towards the surface, receiving electromagnetic return signal reflected at the surface, mixing the return signal with the transmit signal to provide an IF signal and determining the distance based on the IF signal. The step of determining the distance further comprises the steps of dividing the IF signal into a plurality of frequency portions where each frequency portion corresponds to a frequency interval of the transmit signal, identifying frequency portions being affected by narrow-band interference and determining the distance based on frequency portions being unaffected by narrow-band interference.
According to a second aspect of the invention the objects are also achieved through the use of a radar level gauge for narrow-band interference unaffected measurement of a distance to a surface of a product kept in a tank, which radar level gauge comprises a transmitter configured to transmit electromagnetic transmit signal towards the surface, a receiver configured to receive electromagnetic return signal reflected from the surface, a mixer configured to mix the transmit signal and the return signal to provide an IF signal and processing circuitry configured to determine the distance. The processing circuitry is also further arranged to divide the IF signal into a plurality of frequency portions where each frequency portion corresponds to a frequency interval of the transmit signal, identify frequency portions being affected by narrow-band interference and determine the distance from frequency portions being unaffected by narrow-band interference.
By dividing the IF-signal into frequency portions corresponding to frequency intervals of the transmit signal, frequency intervals where narrow-band interferences are present may be detected. By determining the distance to be measured by unaffected parts of the frequency spectrum a more accurate and/or robust measurement may be provided through the removal of erroneous measurements from an average of several measurements.
Furthermore faulty parts which may create microwave-resonances in the antenna or other parts of the waveguide may be identified as periodic narrow-band disturbances. If such a disturbance is encountered the entire radar level gauge may be replaced, and this may save cost and may also save time during maintenance fault tracing.
Moreover if a product, kept in a tank, which the distance is to be measured to exhibits/displays a turbulent surface, the amplitude of the return signal may be low due to the electromagnetic signals being reflected in other directions than the antenna. Some frequency portions may then produce improved measurement results than other frequency portions, which may provide a correct measured distance even under these conditions. In one embodiment of the invention a representative quantity is calculated from each frequency portion and disturbed frequency portions are identified based on this representative quantity. By calculating a quantity for each frequency portion there may be provided a simple way to compare frequency portions, thereby enabling identification of frequency portions which comprises narrow-band interference. For example, deviating representative quantities, which differ more than a predefined amount from a majority of the representative quantities, can be determined, and disturbed frequency portions can be identified as frequency portions associated with these deviating representative quantities.
In order to determined deviating representative quantities, the representative quantities may be sorted in order of size, and a predefined number of the largest and a predefined number of the smallest representative quantities may be identified. In this case, the most diverging representative quantities may be excluded to ensure a correct determined distance. As an example, less than or equal to half the number of representative quantities may be identified as being affected by narrow-band interference, which may keep sufficient information while maintaining a high measurement accuracy.
A comparison of each representative quantity with all other representative quantities and identifying those differing more than a predetermined amount, such as one percent, may provide additional measurement accuracy. Another option is to identify representative quantities differing more than one percent from an average of the representative quantities.
Several ways could be used to compare representative quantities, for example the difference between each representative quantity may be calculated and used, or the representative quantities may just be compared relatively.
According to another embodiment of the invention each of the plurality of frequency portions is processed to provide a frequency spectrum from each of the plurality of frequency portions. The representative quantity may then be calculated as an energy measure from each frequency spectrum.
By first processing each frequency portion to provide a frequency spectrum, and then calculating the energy from each spectrum there may be provided a way to identify frequency portions comprising narrow-band interferences. For example differences between frequency portions may become apparent as differences in energy compared between frequency spectrums.
Alternatively, each frequency portion maybe processed to calculate the representative quantity as a measure of the distance associated with each frequency portion.
Using a measure of the distance as the representative quantity is advantageous as the distance can then be determined as an average of distance measures being unaffected by narrow-band interference. Providing the distance as an average of a plurality of approximated distance which are unaffected by narrow-band interference may improve the measurement accuracy. Of course, this may be useful also if a different representative quantity is used.
Each frequency portion should have a sufficient number of samples to allow a determination of a distance to the surface. In some situations, two frequency portions may be sufficient, while other application may require four or even more frequency portions.
According to one embodiment of the invention the step of transmitting the electromagnetic transmit signal includes, transmitting the electromagnetic transmit signals as a plurality of frequency intervals. Dividing the transmit signals into frequency intervals may alleviate the need to divide the IF-signal into a plurality of frequency portions where each frequency portion corresponds to a frequency interval of the transmit signal, thereby a method which is easier to implement may be provided.
Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.